ADP1621ARMZ-R7 Analog Devices Inc, ADP1621ARMZ-R7 Datasheet - Page 18

• Manufacturer Part Number: ADP1621ARMZ-R7
• Description: IC CTRLR DC/DC PWM STEPUP 10MSOP
• Manufacturer: Analog Devices Inc
• Type: Step-Up (Boost)r

Specifications of ADP1621ARMZ-R7

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Current - Output: 1A
• Frequency - Switching: 100kHz ~ 1.5MHz
• Voltage - Input: 2.9 ~ 5.5 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Primary Input Voltage: 5.5V
• No. Of Outputs: 1
• Output Current: 1A
• No. Of Pins: 10
• Operating Temperature Range: -40°C To +125°C
• Msl: MSL 1 - Unlimited
• Frequency Max: 1.5MHz
• Termination Type: SMD
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With: ADP1621-EVALZ - BOARD EVALUATION FOR ADP1621
• Voltage - Output: -
• Power - Output: -
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Lead free / RoHS Compliant
• Other names: ADP1621ARMZ-R7; ADP1621ARMZ-R7TR

ADP1621
R
represents the external current-sense resistor.
Solving for R
Keep in mind that the above inequality is a function of both
ADP1621 parameters and off-chip components, the values of
which vary from part to part and with temperature. Select R
ensure current-loop stability for all possible variations.
After accounting for parameter variations, use values of R
are as close to the calculated limit as possible because excessive
slope compensation reduces the benefits of current-mode control
and increases the “softness” of the current limit, as discussed in the
Current Limit section. Given a typical peak slope-compensation
current of 70 μA, R
at the CS pin is typically clamped at 116 mV. It is also recom-
mended that R
than 1.6 kΩ, the parameters in Equation 34, such as R
can be adjusted such that R
In conclusion, the value of R
CURRENT LIMIT
The current limit in the ADP1621 limits the peak inductor
current and is achieved by the COMP voltage clamp. The peak
inductor current, I
where V
V
n is the current-sense amplifier gain (typically 9.5), I
peak slope-compensation current (typically 70 μA), R
slope-compensation resistor, D is the duty cycle, f
switching frequency, t
190 ns), and R
lossless current sensing, R
of the external power MOSFET. Otherwise, R
external current-sense resistor.
The current limit in the ADP1621 is a “soft” current limit.
When the inductor current reaches the I
Equation 35, the duty cycle decreases, and the output voltage
drops below the desired voltage. The I
then increases in response to the smaller duty cycle, D. The
larger the slope-compensation resistor, R
on I
in a “soft” current limit for the ADP1621. Use values of R
as close as possible to the calculated limit derived from
Equation 34. If high-precision current limiting is required,
consider inserting a fuse in series with the inductor.
Also, keep in mind that the current limit is a function of both
ADP1621 parameters and off-chip components, the values of
I
DSON
L
COMP,ZCT
,
PK
L,PK
R
, of the external power MOSFET. Otherwise, R
S
=
>
for an incremental decrease in D. This behavior results
COMP,CLAMP
V
is the COMP zero-current threshold (typically 1.0 V),
R
CS
COMP
×
S
(
V
S
gives the slope-compensation criterion:
CS
,
CLAMP
OUT
be greater than 20 Ω. If the calculated R
is the current-sense resistor. In the case of
is the COMP clamp voltage (typically 2.0 V),
S
+
L,PK
2
should not exceed 1.6 kΩ because the voltage
V
n
×
D
−
I
, is given by
OFF,MIN
SC
−
V
V
,
PK
COMP
IN
CS
×
)
S
×
is the minimum off time (typically
f
is equal to the on resistance, R
is less than 1.6 kΩ.
S
,
SW
R
(
ZCT
1
should be 20 Ω ≤ R
CS
−
×
t
L
OFF
−
,
1
MIN
I
−
SC
L,PK
t
×
OFF
,
PK
f
L,PK
SW
S
limit in Equation 35
, the larger the effect
,
×
)
MIN
limit given in
R
CS
S
×
represents the
×
S
f
SW
D
SW
≤ 1.6 kΩ.
CS
CS
is the
, f
SC,PK
S
S
is greater
S
SW
is the
that are
, and L,
is the
S
DSON
that
S
(34)
(35)
Rev. A | Page 18 of 32
to
,
which vary from part to part and with temperature. If lossless
current sensing is used, consider that the on resistance of a
MOSFET typically increases with increasing junction temperature.
The peak inductor current limit also limits the maximum load
current at a given output voltage. The maximum load current,
assuming CCM operation, is given by
If the load current exceeds I
below the desired voltage.
LIGHT LOAD OPERATION
Discontinuous Conduction Mode
With light loads, the average inductor current is small, and,
depending on the converter design, the instantaneous inductor
current may reach 0 during the time when the MOSFET is off.
This mode of operation is termed discontinuous conduction
mode. The condition for entering discontinuous conduction
mode in a boost converter is
When the instantaneous inductor current reaches 0 during the
cycle, the inductor ceases to be a current source, and ringing
can be observed in the waveforms of the MOSFET drain voltage
and the inductor current. The frequency of the ringing is the
resonant frequency of the inductor and the total capacitance
from the SW node to GND, which includes the capacitances of
the MOSFET and diode, and any parasitic capacitances from
the PCB. While adding a resistive element, such as a snubber, to
the system further dampens the resonance, it also decreases the
efficiency of the regulator.
Pulse-Skipping Modulation
The ADP1621 features circuitry that improves the converter
efficiency and minimizes power consumption with no load or
very light loads. When the COMP voltage drops below V
(typically 1.0 V), which can occur at sufficiently light loads, the
MOSFET is powered off until the FB voltage drops below 1.215 V.
Then, the error amplifier drives the COMP voltage higher, and
the converter resumes switching when the COMP voltage rises
above the V
the output capacitor supplies current to the load.
With light loads, the COMP voltage hovers around 1.0 V, and
short periods of switching are followed by long periods of the
MOSFET being powered off. This pulse-skipping modulation
operation improves converter efficiency by reducing the number of
switching cycles and therefore reducing the gate drive current and
the switching transition power loss.
⎛
⎜
⎜
⎜
⎜
⎜
⎝
I
LOAD
V
COMP
I
,
MAX
LOAD
,
CLAMP
=
<
COMP,ZCT
n
(
−
1
V
−
V
IN
COMP
2
D
×
×
)
D
×
L
voltage. While the MOSFET is powered off,
,
R
ZCT
×
×
CS
(
f
1
−
SW
−
1
D
I
−
LOAD,MAX
SC
)
t
,
OFF
PK
,
×
MIN
R
, the output voltage drops
S
×
×
f
D
SW
−
2
V
×
IN
f
SW
×
D
×
L
COMP,ZCT
⎞
⎟
⎟
⎟
⎟
⎟
⎠
(36)
(37)